1. Field of the Invention
The present application relates to a positive temperature coefficient (PTC) composition, and a resistive device and an LED illumination apparatus using the PTC composition.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Because the resistance of conductive composite materials having PTC characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event, occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (i.e., trip) to decrease the current.
The conductivity of the PTC conductive composition depends on the content and type of the conductive fillers dispersed therein. In general, carbon black having rough surfaces can be well adhered to polyolefin, so it provides better resistance repeatability. The over-current protection devices applied to computing, communication or consumer products give weight to resistance repeatability performance, and therefore the conductive ceramic filler in the polymer often uses carbon black. Because the interacting forces among carbon black particles are strong, high density polyethylene (HDPE) is usually used as polymer matrix. However. HDPE has high melting point, resulting in that the PTC composition does not easily trip at a low temperature, and therefore it is not suitable for low-temperature trip applications. Moreover, even if using low-temperature trip polymeric material, the resistance increase after trip may be not sufficient in the case of the use of carbon black. For example, the resistance after trip may be only 100 times the initial resistance, and therefore the PTC composition still needs to be improved.